Remember that the OSPi’s power supply is 24 Volts AC. It derives it’s power and the power for the Raspberry Pi from this base supply. Most irrigation transformers provide about 1 – 1.25 Amps AC which is sufficient to run the electronics and switch two sprinkler valves (Master and one channel). Most fans for electronics use are designed to run from a DC supply. The DC for the electronics is provided by a LM2596 chip which, why adequately powerful to provide enough power, has been implemented on the board specifically to provide a fairly small amount of DC power.
There is no heat sink on the LM2596 so heat generation is a consideration. Further the electrical noise generated by many fans may cause problems for the Pi. Given this and the limited overall power available “within the box” I would recommend that any extra fans are powered by their own, separate power supply.